Kesterite Films Processed with Organic Solvents: Unveiling the Impact of Carbon-Rich Fine-Grain-Layer Formation on Solar-Cell Performance

Solution-processed kesterite (copper zinc tin sulfide [CZTS]) solar cells attract significant attention owing to their low cost, ease of large-scale production, and earth-abundant elemental composition, which make these devices promising to fulfill the ever-increasing demand of the photovoltaic (PV) industry. Compared to the performances of expensive vacuum-based techniques, colloidal nanocrystal kesterite solar cells garner substantial interest due to their economical and rapid processing. Led by the hot-injection method, organic solvent-based techniques are widely adopted to realize CZTS nanocrystal inks. With organic solvents, ligand-stabilized nanoparticles are formed leading to dispersive and homogenous kesterite inks. However, the presence of carbon-rich ligands around the nanocrystal surface often leads to the formation of a fine-grain layer that is rich in carbon content. The organic ligands decompose into amorphous carbon residues during a high-temperature annealing process and hinder the grain growth process. The carbon-rich fine-grain (CRFG) layer generally poses a negative influence on the PV performance of the kesterite solar cell; however, few reports maintain their disposition about CRFG as innocuous. In this review study, a detailed discussion on CRFG is presented, aiming to understand the insights about its formation and impact on the device's performance.

Hot injection with organic solvents is a prevalent technique to fabricate kesterite nanofilms. However, this method leads to the formation of a carbon-rich fine-grain (CRFG) layer beneath the kesterite film. Decomposed carbon matrices in the CRFG layer can account for enhanced series resistance andexcess grain boundaries. Carbon-rich ligands are removed through ligand-exchange/stripping techniques from the surface of kesterite nanocrystals.image (c) 2024 WILEY-VCH GmbH